The present invention relates generally to powder processing technology. More specifically, the present invention relates to apparatuses for grinding or comminuting and for classifying particulate material and employing gravity to more evenly feed the material through the apparatus.
The use of classifying and grinding apparatus is known and important to the production of many items such as pharmaceuticals, chemicals, food products, cosmetics, powder coatings, toners, plastics and paints. The trend towards the use of finer powders (smaller than 50 microns) in certain applications has led to the development of combination mills, which integrate the processes of grinding and air classification into a single circuit. In this type of apparatus, ground product is continuously discharged when it reaches the desired fineness, while material that is still too coarse continues to be ground.
Reducing the percentage of fines at a desired top size and increasing the product yield has been the focus of many powder processing applications. These industrial demands have become even more crucial for powder coating processing due to the need for fine powder coatings, such as in thin film and automotive applications.
Conventional mills have a configuration typified by U.S. Pat. No. 3,285,523 to Duyckinck et al. These mills apply high speed impact for size reduction and utilize a continuous internal recirculation of material to reduce oversize material which has not achieved the desired particle size. Classification is typically achieved using an integrated air classifier in the mill where forced air from below lifts ground material out of the grinding zone and circulates it to the classification zone. In the classification zone, particles smaller than the cut size of classification pass through the classifier and are then collected by a product collector. Oversize particles are sent back to the grinding zone.
Traditional milling equipment flows the material to be ground in an upward direction through the milling mechanism and then through the classifier. As a result, gravity works against the direction of flow, and hinders the proper flow of the material through the apparatus. The larger particles are harder to lift through the different parts of the classifying mill and tend to stay in the mill longer than desirable. This causes these particles to heat excessively, which is detrimental when the particles comprise a material that is heat sensitive, such as powdered paint, plastics, polymers, and food products, including chocolate. Additionally, excessive fines tend to be produced when gravity operates to prolong the residence time of the material in the apparatus. Also, as almost all powders are explosive or flammable, the reduction of heat buildup is highly desirable. Traditional classifiers additionally have regions in which the particulate material tends to accumulate and become trapped, which can also increase heating. This reduces the efficiency of the classifiers and can require that the apparatus be stopped completely for cleaning.
As a result of the extra heating, milling temperature control with very high air flow is often required to ensure satisfactory powder production rate and product quality. Particles at elevated temperature that result from impact fusion can clog machinery and result in a low quality output. In addition, the production of powders with excessive fines can cause problems in powder performance and handling: flow properties deteriorate, airborne dust increases, and process loss and waste can become a serious factor. Product quality may also deteriorate in certain applications, such as powder coating applications where the presence of very fine particles may cause paint rub-off.
Accordingly, there is a need for an classifying mill that can more efficiently process the material with reduced heating. The present invention satisfies this need.
The invention relates to apparatuses for grinding and classifying particulate materials. A preferred embodiment of a classifying mill constructed according to the invention has a housing configured for containing a stream of particulate material and a particulate material feed inlet associated with the housing for introducing the particulate material into the housing. A milling assembly is disposed within the housing and configured for grinding the particulate material into fines and oversize particles. A classifier that has a classifier rotor is disposed within the housing and below the milling assembly. At the underside of the classifier is a fines output that is configured for extracting the fines from the classifier. An oversize particle collector is disposed at the underside of the classifier and radially outward from the fines output and is configured for extracting the oversize particles from the housing. The housing is oriented substantially upright, and the classifying mill is arranged such that the particulate material stream extends downward from the feed inlet, through the milling assembly, and subsequently through the classifier.
The milling assembly includes a long-gap type milling assembly with a milling rotor, which has a radially outward portion extending to adjacent the housing, which preferably includes a plurality of beater plates. The outward portion defines a milling gap between the outward portion and the housing. Also, the rotor and the milling portion of the housing are configured for grinding the particulate material in the milling gap upon rotation of the rotor with respect to the housing. The milling portion of the housing of this embodiment includes a removable lining with ridges configured for increasing the grinding of the particulate material
A particle return manifold is connected to return the oversize particles from the oversize particle collector to the housing to feed them through the milling assembly. A plurality of particle return inlets, each configured for connection to a particle return manifold, is provided in the housing for feeding the oversize particles from the oversize particle collector to the milling assembly.
A drive shaft of one embodiment drives the milling rotor. A bearing assembly supporting the drive shaft is disposed within the milling rotor. Also, this embodiment has a sweeper disposed adjacent the classifier rotor and a first wall portion of the housing, preferably disposed below the classifier rotor. The sweeper is movable with respect to the first wall portion in a sweeper direction along a sweeper path to remove the particulate material from adjacent the first wall portion, preferably centrifugally and around the classifier rotor to reach the oversized particle collector, which is open to the housing at less than the complete circumference thereof. Preferably, the sweeper is rotatable coaxially with respect to the classifier rotor.
The sweeper has an extension that extends into the material stream and which is disposed for moving the particulate material away from the first wall portion. A leading side of the extension faces in the sweeper direction. The leading side in this embodiment is angled away from the first wall portion. The preferred extension comprises a plurality of fins.
In an embodiment, the classifier comprising at least one fluid inlet disposed adjacent the first wall portion and radially inward from the outermost portion of the sweeper. This fluid inlet is configured for increasing the pressure within the sweeper for moving the particulate material centrifugally from the sweeper, and preferably feeds air into the apparatus.
A preferred classifier includes a guide channel connected with a classifying fluid inlet for receiving the classifying fluid, which is preferably air. The guide channel guides the classifying fluid along a channel path, extends substantially coaxially with the classifier rotor, and defines first and second orifices fluidly communicated with the housing. The second orifice is disposed further along the path than the first orifice and is larger than the first orifice. A third is preferably also provided, and is disposed further along the path than second orifice and is larger than the second orifice. The orifices in this embodiment are sizes for feeding the classifying fluid into the housing at about a same rate through each orifice. The preferred guide channel has a substantially constant width in a direction across the flow of the classifying fluid. Additionally, the orifices are oriented at an angle of more than about 45xc2x0 and more preferably more than about 60xc2x0 to the guide channel radius, which preferably extends substantially tangentially to the classifier rotor, and at least one of the orifices is tapered towards the interior of the housing.
In an embodiment of the invention, a blower or eductor is connected to the particle return manifold for blowing a fluid, preferably air, in the particle return manifold past the location at which the oversize particle collector intersects the return manifold, and at an angle thereto. This reduces the pressure in the oversize particle collector to help draw the oversize particles. An adjustable valve is also disposed in the oversize particle collector for controlling the pressure therein, and a feed inlet is connected to the particle return manifold, preferably downstream of the blower or eductor.
The preferred feed inlet is configured and disposed for introducing the particulate material into the milling assembly at an introduction location. The feed inlet also has a loading portion configured for receiving the particulate material at a location below the introduction location for delivery to the introduction location. The loading portion, for example, may comprise a hopper for loading the material.
In an embodiment of a method of grinding and classifying particulate material according to the invention, the material is fed into the apparatus to fall through into a milling rotor in a housing. The milling rotor grinds or comminutes the material into fines and oversize particles. A classifier rotor is used to classify the fines from the oversize particles, and these are separately removed from the apparatus. The oversized particles are returned through a return manifold to fall through the milling rotor for further grinding.